Pressure relief valve construction for railway tank cars and the like

ABSTRACT

Conventional spring type pressure relief valves used on fluid pressure containing tanks function at a single, preset tank internal pressure and provide a fixed orifice and flow area for discharging tank contents. When an essentially conventional spring type pressure relief valve opens and remains open for a significant period of time it may be implied that an emergency condition exists, probably due to excessive heat input as from a fire, and that tank internal pressure exceeds the valve set-to-discharge pressure. This invention identifies the emergency condition by means of the time-dwell of the valve in open position, and actuates a secondary mode of valve operation which provides (a) reduced setto-discharge pressure compatible with tank shell strength at the elevated temperature produced by the fire condition and (b) additionally provides increased discharge capacity to allow sufficient flow to attain a reduced tank internal pressure level.

Q United States Patent us] 3,675,679

Schwartz, Jr. 51 July 11, 1972' 54 PRESSURE RELIEF VALVE 2,644,4807/1953 Earle et al 1 37/469 CONSTRUCTION F OR RAILWAY TANK CARS AND THELIKE Zimary Exrrging-ij-lenry T. Klinksiek tome o n ex [72] Inventor:Frederlch William Schwartz, Jr.,

Clarendon Hills, Ill. [73] Assignee: Jamesbury Corp., Worcester. Mass.[57] ABSTRACT [22] Filed: Dem 29 1970 Conventional spring type presurerelief valves used on fluid pressure containing tanks function at asingle. preset tank inl pp 102.443 temal pressure and provide a fixedorifice and flow area for chm US pp Data discharging tank contents. Whenan essentially conventional spring type pressure relief com'nuauomn'panof 9 I valve opens and remains open for a significant period of timeabandoned it may be implied that an emergency condition exists. probablydue to excesive heat input as from a fire, and that [52] 0.8. CI.137/587, l37/529. Bra/$25545, tank internal pressure exceeds the valvesemodischarge pres 51 l L Cl "6k 31/1: sure. This invention identifiesthe emergency condition by M 492 469 means of the time-dwell of thevalve in open position, and acl e 0 m 4 522 6 A tuates a secondary modeof valve operation which provides (a) reduced set-to-discharge pressurecompatible with tank shell strength at the elevated temperature producedby the tire [56] Re'erenms Cited condition and (b) additionally providesincreased discharge UNlTED STATES PATENTS capacity to alllowlsuflicientflow to attain a reduced tank interna ressure eve. 3,229,713 l/l966Wiegand ..l37l467 p 3.583.422 6ll97l Dach et al ..I37/50S.l4 X 24Claims, l0 Drawing figures PRESSURE RELIEF VALVE CONSTRUCTION FORRAILWAY TANK CARS AND THE LIKE This application is acontinuation-in-part of application, Ser. No. 91,907, filed Nov. 23,1970, now abandoned.

This invention relates, generally, to pressure relief valveconstructions for pressure vessels, such as are used for railway tankcars, tank trucks and the like and it has particular relation to suchconstructions arranged to release the tank contents at a lower pressureand increased volume than it is released initially and normally.

A safety relief valve or other pressure relieving device is provided onpressure vessels to prevent excessive pressure within the vessel. Thisis necessary to insure against rupture of the shell, usually withexplosive force when excessive gas pressures are present. Even in casesinvoiving containment of highly toxic or flammable gases, the lesserrisk is controlled release of the contents of the vessel in preferenceto structural failure of it. Normally such a release through a pressurerelieving device is much smaller in volume and therefore in consequencethan the complete and rapid release which accompanics a tank shellrupture. It is also characteristic of pressure tank failures that largeamounts of potential energy are transformed to propelling forcesscattering physical damage over large areas.

In the majority of cases the structural design of the pressure vesseland the design of the safety relief valve are carefully integrated so asto provide adequate factors of safety. The vessel design isfundamentally related to the vapor pressure of the product to becontained under prescribed conditions. Because the vapor pressure ofliquids varies directly with temperature, the maximum temperature to beencountered in service must be taken into account. Thus temperaturebecomes the basic design parameter for determining product vaporpressure and ultimately, by means of factoring, the design pressures ofthe tank and safety valve. It follows that, in ordinary service and withexposure to normal ambient and other conditions, the safety valve rarelyfunctions at all. Only under exceptional exposures, such as fire, doesthe vapor pressure exceed design conditions.

Not only is the pressure setting of the relief valve of basic importancein relation to tank design pressure but its ability to discharge gas insufficient volume to prevent pressure buildup, even though the valve isopen, is considered in advance. For a specific case it is possible topredetermine the flow capacity required to prevent a pressure increaseand to design a valve to so limit the pressure rise. It is a relatedfact that, as external fire causes the vapor pressure inside the tank torise, the heat must pass through the wall of the tank thereby elevatingthe metal temperature as well. This condition of shell heating is mostevident in areas in which the tank contains gas rather than liquid Thattemperature may exceed l,000 F. It is a corollary fact that, as thetemperature of metals rises above a specific level, the strength of themetal rapidly diminishes. The metal shell of a pressure tank in a fire,then, loses its ability to safely contain the design pressure of thefluid within it.

Among the objects of this invention are: To provide for controlling therelief of pressure from a fluid pressure tank and to take into accountreduced strength of the tank wall resulting from external application ofheat thereto; to utilize the time duration in the open position of apressure relief valve for changing the pressure at which it is relieved;to employ a spring biased relief valve arranged to open at apredetermined pressure and to reduce the compression of the spring orsprings biasing the valve closed as a result of a continuous timeextended opening of the valve; to hold a complete valve assembly insealing engagement with the tank wall and to release such sealingengagement to the control of pressure from auxiliary springs; and toincrease the discharge capacity of the system under emergencyconditions.

A spring biased safety valve is arranged to relieve pressure in a tankwhen it reaches a predetermined value resulting from application ofexternal heat with the consequences of increased internal pressure andreduction in tank wall strength. After extended time-dwell in the openposition the valve actuating modifying means operates to reduce thesafety valve set pressure and to increase the flow area. and thereforethe discharge capacity. The modified safety valve set-pressure may beachieved by either altering the compression of the valve spring or bybringing into operation springs to exert a biasing force to retain thevalve assembly on its mounting plate. Such springs may be designed toexert a force equal to any desired internal balancing pressure.

The flow area is increased by permitting the entire safety valveassembly to be lified, by internal tank pressure and opposed byretaining springs aligned with the mountilg bolts, from its mountingplate. The resulting increase in flow area produces an increaseddischarge rate of the fluid within the tank. Alternatively, or in asupplementary way, the valve ac tuating modifying means opens tankfilling or emptying valves to discharge tank contents at an increasedrate.

In the drawings:

FIG. I is a view, in side elevation, of a typical railway tank car inwhich the present invention can be embodied.

FIG. 2 is a vertical sectional view of a safety valve assemblyconstructed in accordance with a portion of this invention andillustrated in the closed position.

FIG. 3 is a view, similar to FIG. 2, showing the safety valve in theopen position and the system operated to permit the valve to open at alower pressure than the arrangement shown in FIG. 2.

FIG. 4 is a horizontal sectional view taken generally along line 4-4 ofFIG. 5 and shows additional elements of the invention relating to thespring mounting of the safety valve assembly.

FIG. 5 is a vertical sectional view of a modified form of safety valveassembly constructed in accordance with a portion of this invention.

FIG. 6 is a vertical sectional view taken generally along line 6-6 ofFIG. 4.

FIG. 7 is a view, similar to FIG. 6, and shows the cam ring in thealternate position.

FIG. 8 is a view, similar to FIG. 5, and shows the safety valve assemblybodily lifted from the metallic tank shell to increase the dischargearea.

FIG. 9 is a vertical sectional view taken generally along line 9-9 ofFIG. 10 and shown a conventional unloading and loading pressure operatedvalve which can be controlled in accordance with this invention.

FIG. I0 is a vertical sectional view taken generally along line 10-10 ofFIG. 9.

In FIG. I reference character 10 designates, generally, a tank cararranged for railway operation. It includes a tank II for fluid such asliquid or gas. The tank II can be constructed for use with a tank truckor it may represent a stationary tank for containing liquid or gas underpresure. Valves and other devices, indicated generally at I2, areprovided for loading and unloading the tank II and include the safetyvalve construction of this invention. They may be otherwise located onthe tank shell.

In FIGS. 2 and 3 it will be noted that reference character 15 designatesthe metallic tank shell or the mounting plate for valves and devices ofthe tank II and that for illustrative purposes a liquid 16 is shown butit will be understood that the tank may contain a gas under pressure andthat it may be partially liquefied. At any event it will be understoodthat the tank II is intended to contain a fluid which applies asubstantial pressure to the metallic tank shell I5 when the tank II issubjected to external heat such as fire which may be incident toaccident conditions. Mounted on the metallic tank shell 15 is a safetyvalve assembly that is indicated, generally, at 17. It includes a valvehousing 18 provided with a laterally extending mounting flange 19 whichis arranged to be secured in sealed engagement with the metallic tankshell 15 by bolts or studs one of which is indicated at 20. The valvehousing 18 is provided with a discharge opening 21 for the tank II. Avalve plug 22 normally closes the opening 21. Overlying the valve plug22 is a seal disc 23 positioned at the lower end of a guide stem 24which is guided for vertical movement in a support 25 that is centrallylocated with respect to a vent tube 26 that extends upwardly from thevalve housing 18. The metallic tank shell is provided with an opening 28centrally of the valve housing 18. The valve plug 22 is located at theupper end of a valve stem 29 which extends downwardly through theopening 28 and centrally of a valve guide tube 30 which is secured atits upper end to the central portion of the valve housing 18. The valveguide tube 30 extends downwardly through the opening 28 for asubstantial distance. Openings 31 are provided in the wall of the valveguide tube 30 so that its interior is always at the same pressure as theinterior of the tank 11 and flow from the tank interior is relativelyunobstructed through the guide tube 30 and discharge opening 21 to theatmosphere. A coil compression spring 32 is employed for biasing thevalve plug 22 to the closed position. The preset compression of the coilcompression spring 32 is so related to the pressure which is likely tobe developed within the tank 11 as to hold the valve plug 22 in theclosed position when the tank 11 is subjected to normal ambienttemperature. It is only when the pressure within the tank 11 exceedsthis predetermined pressure that the force by the coil compressionspring 32 holding the valve plug 22 closed is exceeded by the force fromtank internal pressure acting to move the valve plug 22 to the openposition. At that time it moves upwardly to permit the release of fluidfrom the tank ll through the discharge opening 21 and the vent tube 26.

It will be observed that the coil compression spring 32 reacts at itsupper end against under side 33 of the valve housing 18. At its lowerend the coil compression spring 32 reacts against the upper side 34 of avalve stem guide or collar 35 which is slidably mounted on the valvestem 29 and within the valve guide tube 30. Apertures 36 are provided inthe valve stem guide 38 so that pressure on both sides of it will alwaysbe the same. Below the valve stem guide 35 there is an abutment nut 37.Between the underside of the valve stem guide 35 and the u per side ofthe abutment nut 37 there is normally positioned a clevis 38 which holdsthe valve stem guide 35 in spaced relation to the abutment nut 37 andthe coil compression spring 32 under predetermined tension.

A basic element of this invention is the identification of the emergencyconditions of high internal pressure in tank 11 due to agreater-than-normal heat input which also produces heating of the tankshell 15 and as a consequence a reduced ability of the tank 11 to safelycontain internal pressure. The detection activation assembly 45 isprovided for this purpose. The function of the detection activationassembly 45 is to accept normal short duration openings of the safetyvalve plug 22 without, in any way, altering the normal function of thesafety valve. In the case of an extended duration opening of the safetyvalve plug 22, however, the detection activation assembly 45 functionsto permit tank internal pressure to enter piping system 64 of FIGS. 2and 3, piping system 7! of FIGS. 4, 5, and 8, and/or piping system 109of FIG. 9. The introduction of tank internal pressure to any one or allof such piping systems and, as a consequence, to actuators 41, 85, and100 alters the mode of operation of the safety valve 17 permitting thesafety valve 17 to open at a reduced internal tank pressure and with anincreased flow area resulting in a higher rate of discharge of fluidunder pressure contained in the tank.

The detection activation assembly 45 illustrated is only one effectivetiming-actuation device. There are several commercially availabledevices which perform like function. These may include devices operatedby mechanical, electrical, or fluid pressure means and any of these maybe employed satisfactorily to perform the dual functions of identifyinggreater than normal extended time the plug 22 remains in the openposition and of introducing tank internal pressure to the several andvarious piping systems mentioned earlier.

The detection activation assembly 45 includes a suitably sealed firstpiston 55 that is slidably mounted in a cylinder 56 which is provided ina housing 44. The diameter, and therefore the pressure area, of thepiston 55 must be smaller than the area of the plug 22 for reasons laterexplained. An extension 57 of the valve stem 29 is threaded into thefirst piston 55 so that it moves conjointly with the valve stem 29.Ports 5! are provided in the housing 44 to place that portion of thecylinder 56 above the first piston 55 in communication with the interiorof the tank 11. Below the first piston 55 and slidably mounted in thelower end of the cylinder 56 is a suitably sealed second piston 59. Acoil compression spring 60 is interposed between the pistons 55 and 59and spacer struts 61 from the first piston 55 hold the pistons inpredetermined spaced relation at a minimum distance. It will be notedthat the lower end of a vent tube 52 is connected at 62 to the portionof the cylinder 56 between the pistons 55 and 59 so that this space isalways at atmospheric pressure. Normally the second piston 59 isinterposed between ports 63 which may be located on opposite sides ofthe cylinder 56.

As the plug 22 of the safety valve 17 opens in response to normal shortduration tank internal pressure increases, the valve stem 29, valve stemextension 57 and the first piston 55 moves conjointly relieving thepreset compression of the coil compression spring 60 and moving thespacer stiuts 61 out of a position in which they positively restrain themovement of the second piston 59. Under normal short duration openingsof the plug 22 there is a subsequent closing of the plug 22 under theinfluence of the compression spring 32 responding to reduced tankinternal pressure as a result of previous venting through the opensafety valve 17.

It will be noted that the interior surface of valve plug 22 is exposedto tank internal pressure and that, with particular reference to FIG. 2,the upper surface of piston 55 is exposed to internal tank pressure.This internal tank pressure is communicated to cylinder 56 throughopenings 58. Piston 55 and valve plug 22 are connected by means of valvestem extension 57 and valve stem 29. Because the pressure area of piston55 is less than the presure area of valve plug 22, net pressure biasagainst valve plug 22 or, with reference to FIG. 2, in the upwarddirection is produced. This net pressure bias permits valve 22 to beopened when the internal tank pressure reaches a predetermined valveactuating pressure. Appropriate adjustment of the compression of coilspring 32 will permit the valve to respond to different actuatingpressures. The desired actuating pressure is, of course, a function ofthe desired circumstances in which the valve of the instant invention isto be employed.

Under abnormal conditions of excessive heat input to the tank 11resulting in sustained higher internal tank pressure the plug 22 opensand remains open for finite and increased time duration. With theinfluences of the compression spring 60 and the spacer struts 61 reducedand removed, the second piston 59 is relatively free to move upwards inFIG. 2 to the position shown in FIG. 3. The remaining force downward dueto the compression spring 60 is overcome by tank internal pressureintroduced to the cylinder space below piston 59 through choke orifice65. The rate of movement of piston 59 is controlled by the choke orifice65 size.

Should a closing of the plug 22 occur prior to complete movement of thepiston 59 the mechanical elements 29, 57, 55, and 61 combine to forcethe piston 59 back to its original position shown in FIG. 2. Should theplug 22 remain open however for a predetermined finite time duration therate controlled upward travel of the piston 59 uncovers ports 63 whichare located at the same elevation but at different radial locationsthrough the cylinder 56 wall. Suitable sealing means are provided forpiston 59 to prevent axial or circumferential leakage of fluid.

As the ports 63 are uncovered by the upward travel of piston 59 tankinternal pressure enters port 63 on the left in FIGS. 2 and 3 and ispermitted to flow through the lower portion of the cylinder 56 belowpiston 59 into piping system 64. Use is made of this pressurized fluidpotential energy to perform actuation functions altering the safetyvalve mode as later described.

As will appear hereinafter, provision is made for changing thecompression of the coil compression spring 32 so as to lower thepressure required to move the valve plug 22 to the open position. It isfor this purpose that the clevis 38 is employed. when it is withdrawn,as shown in FIG. 3, the coil compression spring 32 is allowed to expandto bring the valve stem guide or collar 35 in contact with the abutmentnut 37. In this manner the force exerted by the coil compression spring32 holding the valve plug 22 closed is reduced and it will be opened ata lower pressure within the tank I I.

For controlling the operation of the clevis 38 and withdrawing it fromthe position shown in FIG. 2 to that shown in FIG. 3, a valve actuatingmodifying means, indicated generally at 41, is employed. It includes acylinder 42 which is suitably secured to upper end 43 of a housing 44which is secured to and depends from the lower end of the valve guidetube 30. At its lower end the housing 44 is provided with the detectionactivation assembly that is indicated, generally, at 45. Its functionhas already been explained.

As shown in FIG. 2 the clevis 38 is pivoted at 46 to one end of a pistonrod 47 that extends through opening 48 and 49 in the valve guide tube 30and the upper end 43 of the housing 44. The piston rod 47 is suitablyguided within the cylinder 42 and is arranged to be operated by a piston50 which is slidable in the cylinder 42 on application thereto ofsuitable pressure on the left side. Vent tubes 51 and 52 place thatportion of the cylinder 42 to the right of the piston 50 incommunication with the atmosphere through the metallic shell 15 asindicated in FIG. 2. Pressure applied through piping 64 to the left sideof piston 50 moves the piston 50 to the right withdrawing the clevis 38.

A small diameter bleed tube 66 interconnects the conduit 64 and thelower end of the vent tube 52 to release minor leakage which may takeplace through and around the seals of piston 59 and piston rod 47. Theflow capacity of tube 66 is sufficiently small so as to avoidinterference with the operation of actuator 41 when piston 59 permitstank internal pressure to flow through ports 63 and piping 64 tocylinder 42.

In describing the operation of the safety valve assembly 17 it will beassumed first that the pressure within the tank 11 and against themetallic shell 15 does not exceed the normal pressure that is expectedand for which the coil compression spring 32 is adapted to hold thevalve plug 22 in the closed position shown in FIG. 2. If the pressureincreases due to the tank being subjected, for example, to external heatthen the fluid pressure applied to the valve plug 22 may be such as toovercome the biasing action of the coil compression spring 32. The valveplug 22 then opens to relieve the pressure by allowing the flow of fluidthrough the discharge opening 21. The clevis 38 moves upwardly alongwith the valve stem 29 as does the first piston 55. The spring 60 actsto hold the second piston 59 in the closed position. However, internaltank pressure is applied through the orifice choke 65 to the lower sideof the second piston 59 and it tends to rise. Now if the pressure withinthe tank is relieved by opening of the valve plug 22, the coilcompression spring 32 may be of sufficient strength to reclose the valveplug 22. As previously explained this forces piston 59 to its originalposition insuring that no flow passes through ports 63, thus preventingthe modification of normal safety valve operation. Should the heat inputincreasing tank internal pressure be sufficient to hold the plug 22 openfor a sufiicient length of time to permit piston 59 to move clear ofblockage of ports 63, actuator 41 is activated by fluid pressure toremove clevis 38 to the position shown in FIG. 3. Thereupon the valvestem guide or collar 35 moves downwardly until it engages the upper sideof the abutment nut 37. Con sequently the coil compression spring 32 iscapable of exerting only a lesser closing force against the valve plug22 with the result that a lower internal tank pressure is sufficient toeffect opening of the valve plug 22. This corresponds to a lowering ofthe mechanical strength of the metallic tank shell due to the continuedapplication thereto of external heat.

It will be understood that the foregoing operation of the safety valveassembly 17 is intended to take place only under emergency conditions.After they pass, the system is manually reset. However, it will beunderstood that the internal tank pressure has not been allowed to buildup to such an extent that the metallic tank shell 15 itself is ruptured.

The foregoing elements of the system lower tip set-todischarge pressureto a level compatible with the reduced tank strength at elevatedtemperature. With a substantial input of heat from an external fire itis also important to increase the area available to discharge the tankfluid contents. At high levels of heat input the rate of fluidvaporization, and consequently the tank internal pressure, increase morerapidly than can be accommodated by the restricted flow through thedischarge opening 2I of the safety valve. If additional area is notprovided the tank internal pressure will continue to increase beyondsafe levels even though the plug 22 is continuously held open and theset-to-discharge pressure has been reduced.

FIGS. 9 and 10 show how the discharge area for the tank 11 can beincreased to vent sufficient quantities of the fluid contents to theatmosphere to prevent rupture of the metallic tank shell 15. There isillustrated here, generally, at 100 unloading or loading pressureoperated valve means of conventional construction. It may be locatedover an opening 101 and suitably sealed to the outside of the metallictank shell or the valve mounting plate 15. There is provided a ballvalve I02 which is illustrated in the open position. Here it places theopening 101 in communication with an outlet part 103 to the atmosphere.A shafi 104 is provided through the valve housing to rotate the ballbetween open and closed positions and to which is secured a crank arm105. The crank arm 105 is connected by a link 106 to a piston 107 whichis slidable in a cylinder 108. A conduit 109 is arranged to connect thecylinder 108 to either the conduit 64 or the conduit 7l depending onwhich construction is employed. A manual override of the valve actuatingmechanism, I09, I08, 107, I06, 105, and 104 is provided so that manualopening and closing of the ball valve may be used in the normal way forloading and unloading operations. In operation, when the tank internalpressure is applied either individually or in combination to the valveactuating modifying means 41 or the unloading and loading pressureoperated valve means is operated to the position shown in FIGS. 9 and10. The operation of the detection activation assembly 45 is employed toidentify emergency conditions requiring alternative mode operation ofthe safety valve 17 in an identical manner as previously described. Uponrelease of tank internal pressure by piston 59 to ports 63 and then topiping system 64, through a direct connection between piping system 64and piping system 109 such pressure is introduced to cylinder 108 toopen the ball valve to the position shown in FIGS. 9 and 10. Thus, inaddition to the discharge area provided at discharge opening 2i ofsafety valve I7 and discharge area additionally provided in the mannerlater to be described as opening 28 in FIGS. 5 and 8, the pressureoperated valve means 100 provides for further increase in the dischargearea and further lessening of the likelihood that the metallic tankshell 15 will be ruptured.

In FIGS. 4, 5, 6, 7, and 8 there is illustrated another form of theinvention in which the safety valve assembly 17 is employed with certainmodifications. Here it will be observed that the valve actuatingmodifying means 41 in the form illustrated in FIG. 2 is omitted.Normally the coil compression spring 32 is extended to the fullestextent with the valve stem guide 35 bearing against the abutment nut 37.The detection activation assembly 45 is identical to that shown in FIGS.I and 2 but the piping system is reidentified from 64 to 7I todistinguish the alternative system from the system employing assembly 41previously described.

In the embodiment shown in FIGS. 4, 5, 6, 7, and 8 the entire safetyvalve assembly I7 is arranged to be lifted upwardly for the purpose ofincreasing the discharge area for the pressurized fluid within the tank1 I to be released to atmosphere.

In particular the mounting flange I9 is arranged to be moved upwardly tothe position shown in FIG. 8. For this purpose mounting bolts 73 areemployed. As shown in FIG. 4 four mounting bolts 73 can be used. Themounting bolts 73 extend through the mounting flange l9, the metallictank shell or mounting plate 15 and through spacer means in the form ofa cam ring 74. interposed between the metallic tank shell or mountingplate 15 and the cam ring 74 are tubular spring guide spacers 75 withinwhich coil compression springs 76 are located. The upper end of eachcoil compression spring 76 bears against inside 77 of the metallic tankshell or mounting plate I and the upper side 78 of a lower end closure79 of the respective spring guide spacer 75.

When the safety valve assembly 17 is installed, as shown in FIG. 5, nuts73' are tightened on the mounting bolm 73 to such an extent that themounting flange I9 is held in sealing engagement against the upper sideof the metallic tank shell or mounting plate 15. The springs 76 are heldcaptive within the tubular spring guide spacer 75 but, at this time,have no effect on the upward movement of the mounting flange 19 toincrease the discharge area from the tank 11.

In order to permit the coil compression springs 76 to take control ofthe opening movement of the mounting flange l9 provision is made forpivoting the cam ring 74 from the position shown in FIG. 5 to theposition shown in FIG. 8. For this purpose a lateral flange 81 isprovided on the lower end of the housing 44 to support the cam ring 74.The cam ring 74 pivots about the safety valve center line using thehousing 44 as a rotational guide. An arm 82 extends radially from thecam ring 74 shown more clearly in FIG. 4. One end of a pair of links 83is connected to the distal end of the arm 82 while the other end isconnected to a piston rod 84 which forms a part of valve actuatingmodifying means, indicated generally at 85. This means includes acylinder 86 within which a piston 87, connected to the piston rod 84, isslidable. A vent 88 connects the lower chamber of cylinder 86 in FIG. 4to the atmosphere. A small diameter bleed tube 90 is provided with asimilar purpose to that of bleed tube 66 in FIG. 2, previouslydescribed. The cylinder 86 may be supported by an arm 89 which extendsfrom the underside of the housing 44 and suitably carries the cylinder86 at its distal end or by other suitable means.

As shown in FIG. 4 cylindrical openings 93 are provided in the cam ring74 for receiving the lower ends of the tubular spring guide spacers 75.Inclined cam surfaces 94, FIGS. 6 and 7, interconnect surfaces 95 on theupper side of the cam ring 74 against which the lower ends of thetubular spring guide spacers 75 normally react and the cylindricalopenings 93. Slots 96 are formed in the cam ring 74 to permit the lowerends of the mounting bolts 73 to move conjointly with the tubular springguide spacers 75 to the alternate position.

In operation after the valve plug 22 has opened and remained open for anextended time duration in the manner previously described for thedetection activation device 45 operation pressure is applied through theports 70 and the conduit 7| to the piston 87 in the valve actuatingmodifying means 85. The cam ring 74 then is pivoted as indicated byarrow 97 in FIG. 6 and arrow 98 in FIG. 4 and is moved to the positionshown in FIG. 7. This is accompanied by downward movement of the tubularspring guide spacers 75 into the cylindrical openings 93 to a positionof engagement with surface 99 where the coil compression springs 76 thenconstitute the sole force tending to hold the mounting flange 19 in thesealed position.

Because the combined force of compression springs 76 is designed to beless than the tank internal pressure force on the safety valve assemblyl7 at the set-to-discharge pressure of the safety valve I7 the safetyvalve [7 moves upward through the opening 28 to a position such asillustrated in FIG. 8. This provides discharge area for the pressurizedfluid iii the tank to escape to atmosphere in addition to that areaoriginally provided for this purpose at 21.

The transfer of the closing function from the compression spring 32within the safety valve 17 to the springs 76 restraining the upwardmovement of the entire safety valve asembly I7 permits design of areduced set-to-discharge pressure for the entire safety valve assembly17 and the springs 76 to be the level of tank shell ability to containinternal pressure at elevated temperature.

The openings 31 in the valve guide tube 30 must provide a total flowarea for tank internal pressure escape equal to or greater than the flowarea of the discharge opening 28 in the metallic tank shell 15 and alsogreater than the flow area between the mounting flange l9 and the tankshell or mounting plate 15. Alternatively the guide tube may befunctionally replaced by a series of vertical guide struts similar tothe spacer struts 61 previously described.

What is claimed a new is:

I. A pressure relief valve construction for a fluid pressure containingtank, such as a railway tank car, comprising: means providing adischarge opening for said tank, valve means adapted to be subject totank pressure for closing said opening, biasing means for holding saidvalve means closed until a predetermined fluid pressure is appliedthereto whereupon it is opened, fluid flows through said dischargeopening, and said fluid pressure is reduced, and means responsive to thetime duration said valve means is in an open position for changing thepressure at which said fluid pressure is released from said tank.

2. The pressure relief valve construction called for by claim I whereinsaid means responsive to opening of said valve means includes: valveactuating modifying means, and valve means responsive to tank pressurefor applying said pressure to said valve actuating modifying means foroperating the same.

3. The pressure relief valve construction called for by claim I whereinsaid means providing a discharge opening includes a flange adapted to besecured to a wall of said tank, a perforate sleeve extends from saidflange for positioning in said tank, said valve means includes a valvestem extending through said sleeve, and said biasing means reactsbetween said flange and said valve stem.

4. The pressure relief valve construction called for by claim 3 whereincylinder means extends endwise from said sleeve, a first pistonconnected to said valve stem is movable therewith in said cylindermeans, a second piston is slidable in said cylinder means and functionsas a valve to apply tank pressure to valve actuating modifying means toeffect said change in pressure at which said fluid pressure is released,and spring means between said pistons biases them apart.

5. The pressure relief valve construction called for by claim 4 whereinvalve actuating modifying means is connected to said valve stem and iscontrolled by said second piston to vary the connection between saidbiasing means and said valve stem.

6. The pressure relief valve construction called for by claim 5 whereina collar is slidable on said valve stem against which said biasing meansbears, abutment means is secured to said valve stem, and spacer meansbetween said collar and said abutment means is arranged to be withdrawnby said valve actuating modifying means to change the action of saidbiasing means reacting between said flange and said valve stem.

7. The pressure relief valve construction called for by claim 4 whereinorifice means is provided for interconnecting said cylinder means andthe interior of said tank to apply tank pressure to said second pistonat a reduced rate.

8. The pressure relief valve construction called for by claim 3 whereinflange spring means biases said flange into sealing engagement with saidwall of said tank, spacer means is interposed between said flange andsaid flange spring means, and valve actuating modifying means isconnected to said spacer means to shitt the same and vary the forceapplied by said flange spring means to said flange to permit the same tomove away from said tank wall at a predetermined tank pressure and allowfluid discharge therepast.

9. The pressure relief valve construction called for by claim 8 whereinsaid flange spring means comprises a plurality of coil compressionsprings, a mounting bolt is arranged to extend from said flange throughsaid tank wall into said tank and through each compression spring. andsaid spacer means comprises a cam ring interposed between eachcompression spring and the respective mounting bolt.

It]. The pressure relief valve construction called for by claim 9wherein a spring guide encloses each compression spring and is arrangedat one end to react against the inside of said tank wall and at theother end against said cam ring.

11. The pressure relief valve construction called for by claim 9 whereincylinder means extends endwise from said sleeve, a first pistonconnected to said valve stem is movable therewith in said cylindermeans, a second piston is slidable in said cylinder means and functionsas a valve to apply tank pressure to said valve actuating modifyingmeans, spring means between said pistons biases them apart, and meanspivotally mount said cam ring around said cylinder means.

12. The pressure relief valve construction called for by claim 2 whereinat least one loading and unloading pressure operated valve means isprovided for supplying and removing fluid to and from said tank, andmeans is provided for applying said tank pressure to said pressureoperated valve means for opening the same on application of said tankpressure to said valve actuating modifying means.

[3. A pressure relief valve adapted for use in association with apressurized system containing a fluid, said pressure relief valvecomprising: valve means movable from a closed to an open position:biasing means exerting a given magnitude of force against said valvemeans for holding said valve means in a closed position until a givenpressure is developed in said system whereupon said valve means moves tosaid open position; first means responsive to the time duration saidvalve means is in an open position; and second means for changing themagnitude of force exerted against said valve means by said biasingmeans; said first means being operatively connected to said second meansto activate said second means when said valve means is in an openposition for a given time duration.

14. The pressure relief valve defined in claim 13 wherein said firstmeans utilizes the pressure in said system to activate said secondmeans.

15. A pressure relief valve adapted for use in a system containing afluid, said pressure relief valve comprising: valve means movable from aclosed to an open position; biasing means exerting a given magnitude offorce against said valve means for holding said valve means in a closedposition until a given pressure is developed in said system whereuponsaid valve means moves to said open position; first means responsive tothe time duration said valve means is in an open position; second meansfor reducing the magnitude of force exerted against said valve means bysaid biasing means; said first means being operatively connected to saidsecond means to activate said second means when said valve means is inan open position for a given time duration; whereby, when said valvemeans is in an open position for said given period of time said firstmeans activates said second means and a reduced magnitude of force isexerted against said valve means by said biasing means.

16. The present relief valve in claim 15 wherein said first meansutilizes the pressure in said system to activate said second means.

17. A pressure relief valve adapted for use in association with apressurized system containing a fluid, said pressure relief valvecomprising: valve means movable from a closed to an open position;biasing means holding said valve means in a closed position until agiven pressure is developed in said system whereupon said valve meansmoves to said open position; first means responsive to the time durationsaid valve means is in an open position; and, second means for changingthe discharge capacity for release of fluid from said system; said firstmeans being operatively connected to said second means to activate saidsecond means when said valve means is in an o n position for a ven timeduration. I

18. e pressure relie valve defined in claim 17 wherein said first meansutilizes the pressure in said systen to activate said second means.

19. A pressure relief valve adapted for use in association with apressurized system containing a fluid, said pressure relief valvecomprising: valve means movable from a closed to an open position;biasing means holding said valve means in a closed position until agiven pressure is developed in said system whereupon said valve meansmoves to said open position to provide a given discharge capacity forrelease of fluid from said system; first means responsive to the timeduration said valve means is in an open position; and, second means forproviding a greater discharge capacity for release of fluid from saidsystem; said first means being operatively connected to said means toactivate said second means when said valve means is in an open positionfor a given time duration.

20. The pressure relief valve defined in claim 18 wherein said firstmeans utilizes the pressure in said system to activate said secondmeans.

21. A pressure relief valve construction adapted for use in associationwith a pressurized system containing a fluid, said pressure relief valveconstruction comprising: valve means movable from a closed to an openedposition; biasing means exerting a given magnitude of force against saidvalve means for holding said valve means in a closed position until agiven pressure is developed in said system whereupon said valve meansmoves to said open position to provide a given discharge capacity forrelease of fluid from said system; first means responsive to the timeduration said valve means is in an open position; second means forchanging the magnitude of force exerted against said valve means by saidbiasing means; and third means for changing the discharge capacity forrelease of fluid from said system; said first means being operativelyconnected to said second and third means to activate said second andthird means when said valve means is in an open position for a giventime duration.

22. The pressure relief valve defined in claim 13 wherein said firstmeans utilizes the pressure in said system to activate said second andthird means.

23. A pressure relief valve adapted for use in association with apressurized system containing a fluid, said pressure relief valvecomprising: valve means movable from a closed to an open position;biasing means exerting a given magnitude of force against said valvemeans for holding said valve means in a closed position until a givenpressure is developed in said system whereupon said valve means moves tosaid open position to provide a given discharge capacity for release offluid from said system; first means responsive to the time duration saidvalve means is in an open position; second means for reducing themagnitude of force exerted against said valve means by said biasingmeans; and third means for providing a greater discharge capacity forrelease of fluid from said system; said first means being operativelyconnected to said second and third means to activate said second andthird means when said valve means is in an open position for a giventime duration.

24. The pressure relief valve defined in claim 23 wherein said firstmeans utilizes the pressure in said system to activate said second andthird means.

I i t i l

1. A pressure relief valve construction for a fluid pressure containingtank, such as a railway tank car, comprising: means providing adischarge opening for said tank, valve means adapted to be subject totank pressure for closing said opening, biasing means for holding saidvalve means closed until a predetermined fluid pressure is appliedthereto whereupon it is opened, fluid flows through said dischargeopening, and said fluid pressure is reduced, and means responsive to thetime duration said valve means is in an open position for changing thepressure at which said fluid pressure is released from said tank.
 2. Thepressure relief valve construction called for by claim 1 wherein saidmeans responsive to opening of said valve means includes: valveactuating modifying means, and valve means responsive to tank pressurefor applying said pressure to said valve actuating modifying means foroperating the same.
 3. The pressure relief valve construction called forby claim 1 wherein said means providing a discharge opening includes aflange adapted to be secured to a wall of said tank, a perforate sleeveextends from said flange for positioning in said tank, said valve meansincludes a valve stem extending through said sleeve, and said biasingmeans reacts between said flange and said valve stem.
 4. The pressurerelief valve construction called for by claim 3 wherein cylinder meansextends endwise from said sleeve, a first piston connected to said valvestem is movable therewith in said cylinder means, a second piston isslidable in said cylinder means and functions as a valve to apply tankpressure to valve actuating modifying means to effect said change inpressure at which said fluid pressure is released, and spring meansbetween said pistons biases them apart.
 5. The pressure relief valveconstruction called for by claim 4 wherein valve actuating modifyingmeans is connected to said valve stem and is controlled by said secondpiston to vary the connection between said biasing means and said valvestem.
 6. The pressure relief valve construction called for by claim 5wherein a collar is slidable on said valve stem against which saidbiasing means bears, abutment means is secured to said valve stem, andspacer means between said collar and said abutment means is arranged tobe withdrawn by said valve actuating modifying means to change theaction of said biasing means reacting between said flange and said valvestem.
 7. The pressure relief valve construction called for by claim 4wherein orifice means is provided for interconnecting said cylindermeans and the interior of said tank to apply tank pressure to saidsecond piston at a reduced rate.
 8. The pressure relief valveconstruction called for by claim 3 wherein flange spring means biasessaid flange into sealing engagement with said wall of said tank, spacermeans is interposed between said flange and said flange spring means,and valve actuating modifying means is connected to said spacer means toshift the same and vary the force applied by said flange spring means tosaid flange to permit the same to move away from said tank wall at apredetermined tank pressure and allow fluid discharge therepast.
 9. Thepressure relief valve construction called for by claim 8 wherein saidflange spring means comprises a plurality of coil compression springs, amounting bolt is arranged to extend from said flange through said tankwall into said tank and through each compression spring, and said spacermeans comprises a cam ring interposed between each compression springand the respective mOunting bolt.
 10. The pressure relief valveconstruction called for by claim 9 wherein a spring guide encloses eachcompression spring and is arranged at one end to react against theinside of said tank wall and at the other end against said cam ring. 11.The pressure relief valve construction called for by claim 9 whereincylinder means extends endwise from said sleeve, a first pistonconnected to said valve stem is movable therewith in said cylindermeans, a second piston is slidable in said cylinder means and functionsas a valve to apply tank pressure to said valve actuating modifyingmeans, spring means between said pistons biases them apart, and meanspivotally mount said cam ring around said cylinder means.
 12. Thepressure relief valve construction called for by claim 2 wherein atleast one loading and unloading pressure operated valve means isprovided for supplying and removing fluid to and from said tank, andmeans is provided for applying said tank pressure to said pressureoperated valve means for opening the same on application of said tankpressure to said valve actuating modifying means.
 13. A pressure reliefvalve adapted for use in association with a pressurized systemcontaining a fluid, said pressure relief valve comprising: valve meansmovable from a closed to an open position: biasing means exerting agiven magnitude of force against said valve means for holding said valvemeans in a closed position until a given pressure is developed in saidsystem whereupon said valve means moves to said open position; firstmeans responsive to the time duration said valve means is in an openposition; and second means for changing the magnitude of force exertedagainst said valve means by said biasing means; said first means beingoperatively connected to said second means to activate said second meanswhen said valve means is in an open position for a given time duration.14. The pressure relief valve defined in claim 13 wherein said firstmeans utilizes the pressure in said system to activate said secondmeans.
 15. A pressure relief valve adapted for use in a systemcontaining a fluid, said pressure relief valve comprising: valve meansmovable from a closed to an open position; biasing means exerting agiven magnitude of force against said valve means for holding said valvemeans in a closed position until a given pressure is developed in saidsystem whereupon said valve means moves to said open position; firstmeans responsive to the time duration said valve means is in an openposition; second means for reducing the magnitude of force exertedagainst said valve means by said biasing means; said first means beingoperatively connected to said second means to activate said second meanswhen said valve means is in an open position for a given time duration;whereby, when said valve means is in an open position for said givenperiod of time said first means activates said second means and areduced magnitude of force is exerted against said valve means by saidbiasing means.
 16. The present relief valve in claim 15 wherein saidfirst means utilizes the pressure in said system to activate said secondmeans.
 17. A pressure relief valve adapted for use in association with apressurized system containing a fluid, said pressure relief valvecomprising: valve means movable from a closed to an open position;biasing means holding said valve means in a closed position until agiven pressure is developed in said system whereupon said valve meansmoves to said open position; first means responsive to the time durationsaid valve means is in an open position; and, second means for changingthe discharge capacity for release of fluid from said system; said firstmeans being operatively connected to said second means to activate saidsecond means when said valve means is in an open position for a giventime duration.
 18. The pressure relief valve defined in claim 17 whereinsaid first means utilizes the pressure in said system to activate saidsecond means.
 19. A pressurE relief valve adapted for use in associationwith a pressurized system containing a fluid, said pressure relief valvecomprising: valve means movable from a closed to an open position;biasing means holding said valve means in a closed position until agiven pressure is developed in said system whereupon said valve meansmoves to said open position to provide a given discharge capacity forrelease of fluid from said system; first means responsive to the timeduration said valve means is in an open position; and, second means forproviding a greater discharge capacity for release of fluid from saidsystem; said first means being operatively connected to said means toactivate said second means when said valve means is in an open positionfor a given time duration.
 20. The pressure relief valve defined inclaim 18 wherein said first means utilizes the pressure in said systemto activate said second means.
 21. A pressure relief valve constructionadapted for use in association with a pressurized system containing afluid, said pressure relief valve construction comprising: valve meansmovable from a closed to an opened position; biasing means exerting agiven magnitude of force against said valve means for holding said valvemeans in a closed position until a given pressure is developed in saidsystem whereupon said valve means moves to said open position to providea given discharge capacity for release of fluid from said system; firstmeans responsive to the time duration said valve means is in an openposition; second means for changing the magnitude of force exertedagainst said valve means by said biasing means; and third means forchanging the discharge capacity for release of fluid from said system;said first means being operatively connected to said second and thirdmeans to activate said second and third means when said valve means isin an open position for a given time duration.
 22. The pressure reliefvalve defined in claim 13 wherein said first means utilizes the pressurein said system to activate said second and third means.
 23. A pressurerelief valve adapted for use in association with a pressurized systemcontaining a fluid, said pressure relief valve comprising: valve meansmovable from a closed to an open position; biasing means exerting agiven magnitude of force against said valve means for holding said valvemeans in a closed position until a given pressure is developed in saidsystem whereupon said valve means moves to said open position to providea given discharge capacity for release of fluid from said system; firstmeans responsive to the time duration said valve means is in an openposition; second means for reducing the magnitude of force exertedagainst said valve means by said biasing means; and third means forproviding a greater discharge capacity for release of fluid from saidsystem; said first means being operatively connected to said second andthird means to activate said second and third means when said valvemeans is in an open position for a given time duration.
 24. The pressurerelief valve defined in claim 23 wherein said first means utilizes thepressure in said system to activate said second and third means.